Recovery of precious metal values by cyanidation



Patented Aug. 23, 1949 UNITED STATES ATENT orr c RECOVERY OF- PREC IOUS METALVALUES BY'CYANIDATION Earl' C. Herkenhoff and Norman Hedley, Stamford, Cnn., assignors to American Cyanamid Company, New York, N. Y., a corporationof Maine No Drawing. Application August '10, 1946} Serial No. 689,809

This invention'relates to an improved process for recovering precious metals by cyanidation, and more particularly to the treatment of very slimy gold ores.

Cyanidation is one of the most important ore dressing methods used in the recovery of precious metals, such as gold and silver, from ores containing them. In the ordinary cyanidation process the finely divided ore is treated with a solution of cyanide, such as sodium or calcium cyanide, in the presence of lime and oxygen to dissolve the precious metals in the form of their soluble double cyanides; The solution thus obtained, which is known as the pregnant solution,

' is then filtered from the ore pulp and the precious metals precipitated by the use of finely divided zinc, followed by a second filtration. The zinc precipitation method has been developed to a very high state of metallurgical efiiciency. In fact,

ment. Therefore, low grade slimyores, such as for example, the so-called saprolites, have not been considered hitherto economically treatable by the ordinary cyanidation procedure using zinc precipitation, because the sticky slimes of these ores impose such a load on the filters that the filtration step cannot be carried out economically. Theoretically the continuous counter-current decantation method of treatment may be applied to slimy ores but ordinarily requires a very high bon from the ore pulp with the precipitated precious metal values in the pores of the carbon. The carbon can be used for precipitation without first filtering the ore pulp 'to' produce a clear solution, whichis required in the zinc process. However, thefactivated carbon precipitation process,

6 Claims. (01. 75-7106),

while it has opened up some types of ore to cyanidation which was hitherto untreatable by the zinc precipitation process, is by no means'ideal and has serious drawbacks, These drawbacks arise largely from the method which has. hitherto been used in removing the precious metal bearing carbon from the orepulp. The process employed involves froth flotation in the presence of a collector for carbon, such as hydrocarbon oils. It is possible to obtain a'reasonably high recovery of precious metal by this method, but usually at a very serious sacrifice in gradeof the flotation concentrate due to contamination with readilyfloatablegangue slime and, in some cases, with barren sulfides. It should be remembered that this flotation concentrate must thenbedewatered, which is made difiicult by the high slime content,

I and then processed, usually by pyrometallurgical methods, in order to recover the precious metals.

A fairly high grade of concentrate is necessary in order to keep costs and losses down. Unfortunately, the carbon precipitation process followed by fiotation results in a low grade of concentrate and quite often the recovery of the loaded carbon is not complete, resulting in high residue losses. Another serious drawback lies in the fact that flotation requires a small size of carbon particle. As a result, the concentrate contains relatively finely divided carbon which, in

pyrometallurgical processes, increases the mechanical dust loss when the carbon is smelted.

Still another objection is that the collectors employed for flotation of the carbon tend to greatly reduce theactivity of the carbon and. make it unfit for further precipitation if cycling or counter-current flow of the carbon is employed.

In spite of the above drawbacks cyanidation processes using activated carbon as the precipitant represent an important advance in the treatment of certain precious metal ores or fractions thereof. They are, however, compromises, leaving much to be desired, and of course the drawbacks of the process, which are reflected in highercosts, correspondingly restrict the field of applicability of the process. Certain slimy lowgrade ores cannot be economically treated even with ordinary activated carbon precipitation methods.

The present invention constitutes an improvement in the activated carbon precipitation process which removes the major drawbacks, permitting metallurgy which, under favorable circumstances, is substantially as good as that obtained by zinc precipitation, and at the same time recovers a high grade productwhich can be effec- .sumciently magnetic to permit recovery on or-.

dinary magnetic separators.

The process of the present invention has-many '1 advantages over the known activated carbon precipitating processes in which recovery w a s -ef-. and is veryreadily filtered or de-watered. This .{i'S a great advantage over the recovery of carbon fected by froth flotation. :the first place, the ratio of concentration is much highexyoften. pro: ducing 3% times as high a grade. Secondly, magnetic separation is one of the most positive, rapid v and cheap recovery procedures, using contact; equipment, and requiring a minimum of labor 4 fiowsheets or to adapt fiowsheets to local conditions.

A magnetic recovery of the activated carbon containing the precipitated precious metals follows in general standard magnetic-recovery procedures, and the common types of magnetic separators may be employed without any material change. The possibility of using standard equipment reduces materially the cost of this portion of the process. 7

The concentrate itself is of granular character by froth flotation. It may thenbe shipped directly to. smelters or it may be burned and the 'small 'volumeof residue cyanided or otherwise treated by known methods for handling such conand power cost, and the treatment does notaf fect the activity of the-carbon. A further advantage of the process of the @presenteinizentionrlies in the fact that :it is not neeessaryifto use 11011 'finely divided carbon. Magnetic separation. .is i-ust as effectiye onlargeflcarbon particles as those of flotation size; This introduce-ea further economy, since it isnotnecessary to :cruslsuand size the carbon .careiullya-nd dustlossesimm excessive amounts of fines areeliminated or greatly reduced. At the same theme .of larger carbonparticles hasbeen found tozinvolve nozcmtrepared and then treated with ga iinely, divided magnetic solid .such as. magnetitewfior example, magnetic concentrates, .using .av suitable binder to cause the magnetic particles to .aidhere to the .earbon particles. Any-binder which is not affected by the .cyanidiation pnocedure and does not interfere with precipitation Lbensed, and the present inventionis in nosense -hmited to the-17356 of any particular binder; .fE'rozdinm silicate, however, is preferred, :as. it 'i-sone :of the mosteifec-tive bindersgand at the same very -eheap.'- After the magnetic -materi a=1 been incorporated in the actiwated carbon -tl'ie product "normally dried at a temperature not greatly exceeding the boilingpoint-of water; andre'activated by heating to'the cnstomaryhig-h temperatu-re for a short period of times Another method of incorporating- -magnetic particles in the carbon is 'todmpregnate the carbon with a solution of an iron salt, which is then treated with a reducing agent order toreduce v the salt to metallic iron.

A 'third method is to produce magnetic iron oxide in the carbon bydecomposing an absorbed solution of a suitable ironsalt with heat.

It is an advantage of the present invention that the process steps are quitejrlex ilzi1e. Thus,

for example, the magnetic carbon "may be added either during the dissolution of the precious metals by the cyanide. solution and' after solution is complete. "It is also possibleto add the .carbon in stages or to effect a countercurrent flow, that is to say, the fresh carbonbeing added to the weakest part of the solution. "This flexibility of. procedure makes it jpfossibletolfit the provements of the present invention into existing .cen trates. The presence of the small amount of magnetic material does not interfere in any way with the recovery precious J-netals ,fnomy-the concentrate. V r v Some. low grade precions sores contain small amounts of magnetite or similar .magnetic materials. These, of .counse. would be removed with the magnetic-carbon concentrate and would result in reduction of .gradeof the-latter. lt is therefore. desirable when anore .hasenyconsiderable content of magnetic material, to subject it to a plieliminary magnetic separation .tofremoye such material. Thetailingl immthis pigeliminaxy magnetic separator is then treatedby the process of the presentinvention, .exactlylasis theease with non-magneticbre.

The invention will. be described in. greater'detail in con-Junction with .flollowing specific examples. Although in most'flof the examples technical grade .of black calcium, cyanide was used in accordance with standard cyanidation practice, thereagent consumptidnsane fi p essed as socium cyanide equivalents.

Example 1- a A gold ore from California was used containing a higher proporti'onoi refractory shr nes, and V The prineipa-l mineral constituentswere quartz, sericite,- iron oxides (including hematitcfl'imoni'te andmagnetite) dolomite and minor amounts- 0f pyrite, chalcopyri'te and arsenopyrite the The ore was -;gro1-md to a product having following size-distribution? 7 'Miesh Ter'Cent'Wt.

Tota1 l ce-00 Cyanidation was .carried out by grinding the one :with hydrated lime tov produce :a saturated solution, adding. technical. calcium. cyanide and agitating the pulpfor 2/1 hours at 25% solids with the :usualiaeration. pulpwslsfiltered and washed on. -a laboratory filter a clear, pregnant solution, which. .was then treated with a precipitant in the usual manner and .assayed in order to obtain reference data for the low-grade gold ore from Per Cent Assay, Oz. Per Cent Reagent Con- Product Wt. Au/ton Distrib; sumption of Au I Lbs/Ton Cale Head 100. 0. 220' 100. 00 0.82 NaCN; 'Preg. and Wash 353. 33' 0.057 88.96 8.0 0210.

Solution. Y Residue.' 100. 00 0. 025

A second portion of the same gold ore was likewise ground with lime, the ground pulp thickened and the naturally occurring magnetite, about 0.3% by Weight, removed in a magnetic separator. The pulp was then cyanided with technical calcium cyanide at a density of 47%, additional lime being added and the cyanidation continued for 24 hours under optimum aeration conditions.

At the end of 24 hours 9 lbs/ton of magnetic activated peach pit charcoal of +20 +48 mesh was added. The charcoal was prepared by treating the originally activated charcoal with finely ground magnetite and sodium silicate, followed by drying and reactivation, the proportions being as follows:

. Parts 20 +48 mesh activated peach pit carbon 25 20 mesh magnetite 20 40 B sodium silicate l8 HzQ 8 The product was driedat 110 C. and activated for 8 minutes at 1600 F. V

The pulp was then agitated for a further 24 hours and additional calcium hydroxide wasadded to maintain alkalinity. Thereupon the pulp was subjected to magnetic separation and the magnetic concentrate cleaned once magnetically. The various products were then filtered in the laboratory, washed and assayed. The metallurgical results are as follows:

Per Cent Reagent Per Cent Assay, Oz. I Product wt All/Ton Distal]. COIlSum Fgllgll,

Calc. Head 100. 00 0. 227 100. 00 0.00 NaON. L/Iag. Conc 0.59 a 32.895 85.02 19.7 CaO. Barren andWaslL 281.67 0.0027 3.35 9.0 Mag. 0. Residue 100. 00 0. 025 11. 03

It will be apparent that the magnetic carbon process recovered all but a small portion of the gold that was dissolved by cyanide, and the reagent consumption was moderate, the cyanide Example 2 A series of five tests were carried out with a Nevada having the following composition:

Gold oz. Au/ton 0.13-0.14 Silver Sulfur 0.08 Iron Fe 2.62

6 and containing principally quartz, seiicit', feldspar; kao1in,'iron oxides and some ferromagne's'ian minerals. This ore is too slimy for economical cyanidation by ordinary means.

Test 1.-A test charge of the'ore was ground to 16.5% mesh, and 53.0% 200 mesh with hydrated lime :and the product was thickened by decantation. The naturally occurring magnetite (less than 1.0% of the weight) was removed and the pulp was transferred to a wide-mouth agitating bottle. Additions were made of cyanide and lime and the pulp. at 37% solidsv was agitated for a 24-hour period. Magnetic carbon, (the same preparation as used in Example 1) was then added in the amount of 6.7 lbs. per ton of feed and the pulp wasagitated for an additional 24 hour period, after which the carbon was recovered magnetically and the pulp was filtered and washed. The results obtained; were asQf llows;

was treated exactly as in Test 1. At a pulp density of 37 solids, additions of cyanide, lime and magnetic carbon (same as used in Examplel) were made and the pulp was agitated for a 24- hour period, dissolution and precipitation of the gold on the carbon taking place simultaneously. After 24-hours contact, the carbon was recovered magnetically and the pulp was filtered an washed. The results were asfollows:

. v Per Cent Rca ent Con- Per Cent Assay Oz. Product Distnb. sumption, fAu/Ton of Au Lbs/Ton" Gaic.Head 100. 00 0.140 1 100. 00 0.30 NaCN.

.Mag. Cone. 0.37 32.407. 85. 66 BarrenandW "254.17 0.0020 3.63 8.0 Ca-O. Residue 100.00 0.015 10.71 6.7 Magi'C.

Test 3.-A comparison test was run in which the ore was ground and cyanidedfor a 24-hour period. The pulp was then filtered and Washed and the pregnant solution (plus wash) and residue were submitted for assay. The results were:

thickened and the magnetic portion was removed. The pulp was transferred to a wide-mouth agitating bottle at 37% solids, and was agitated for 48 hours with lime, cyanide and magnetic charcoal prepared from 48 +200 mesh black walnut shell charcoal, magnetite and sodium silicate in the weight proportions after drying of 59% carbon, 35% magnetite and 6% sodium silicate. 1

After 48 hours contact, the magnetic charcoal was recovered magnetically and the pulp was Z filtered-end washed. Therzariou products were submitted iorassam T-heresults. app ear. below.v

. Per Gent 1 Assay- Oz. Reagent-Con 2E. 161 DlStYlb. sum non. I; v "of'Au' Lbsglon on Head 100500 0.142 100100 V no 0054' 221635 J 8589 0-43 NaGN.

. 1 1 5Q 0. 00].- 11 8.1 v 14, &, 03.0.- 100110' 1 020175 12230 '9. M-ag:

' -Percent- Reagent-Corr Percent Assay Oz. Rtodnct. .Distnb. sumption Wt Au/rm ofAiu Lbs/Ton Cale. Head 100. 00- 0.140 100.00 Mag. Cone..." 0.025. 19. 400. 80. 51 0.37 NaCN. Barren Soln. and

W 0.0005 1 1.00 0.9.08.0: Residue' 100:.00; 0.0175 12:49 9.0 Mag. 0.

T e activated maple wood charcoal used waspreparedas follows; sports. of -20. +zoo mesh. ac.- tlvated maple wood charcoal was. treated with Zpantsoi =-2OQ- mesh magnetite, and. 6. parts of 5.0%,. solution oi: 40f. B sodium silicate. 'I-fhe preparation. was driedand; reactivation effected by heating; ion 4. minutes at. 160.0,? E.

It will be noted from the above tests that the recovery of gold was but little below th theoretijeally recoverable values, as. established by Test. 3-. This latter test, of course, was carried outinthe, laboratory as the'ore is-not susceptible to psacticatfiltratiom on a commercial scale. This test. merely shows. the theoretical maximum amount of gold receuenable by: oyanidation.

Example 3' The ore-of Example 1* was used but the procedure' was varied; in that the pulp density was 42% solids and the magnetic carbon was: added with the cyanide and lime during dissolution. The magnetic: carbon used. was prepared; from maple woodcharcoal as follows:

- Parts -11); +100 mesh activated maple wood charcoalmmm. 5 -200 meshmagnetite" 3 40"- sodium silicate 4 Water 3 and" was calculated" to contain, after activation,

55%, carbon, 33%: magnetiteand. 12% sodium silicate.

The results. of. the test. asfollowsz.

' 8 It will. be: noted. .tha t the loss inbarrensolution and wash, although small, is slightly in. excess otthatobtained by the. procedure. of Example 1. This could readily be reduced-by counter-flow of thecarbom Y 1 Examples The procedure of Example 3 was followed except. that the pulp density was 40% solids and thewma-gneticzcarbon was prepared from -20 +48 mesh activated peach pit carbon, sodium silicate and -325 mesh ferrosilicon. The composition of the magnetic carbon after drying, andactjivation was 64%. carbon, 27% ferrosilicon and: 9% sodium silicate. Th metallurgical results are shown in the following table: 1 w

- 1 i 1 1 'Per-Ccnt Reagent Inn-Cent Assay 4 Product vDistrib. Consumption 7 of Au; Lbs/Tom Cale-.Head 100.06 1 0.215 100.00 Y Mag-.Eonc N. 0.61, 29.365. 83.46- 0:53 NaGN'. Barren Solnand 275. 00 0.0020 2. 56 17.5 CaO.

Wash; R8Sid1le. 100.00 i 0.030; 13:98 11.0 Mag. C.

- Example '5 V The. procedure of Example 4. wasfollowedex,- cepiz. that. in; the preparation, at the; magnetic carbon, very fine magnetic black iron oxide was used, which is a by-product from the manufacture of aniline. This product has particle size of approximately- 5 microns or less. The

composition was approximately 62% carbon, 25% blackiron oxideand 13% sodium silicatei The quantity used likewise differed-- slightly from that of; Example. 4, 1 2 lbs. per tonbeing used instead of 11.7 lbs. The metallurgical results appear, in the following table: Y

C I A figm ce t 5 Rms er en ssay is ri onsump Product 5 we. on All/1 103 011m 6011,;

V Lbs.[lon

l v Calc.Had '1 100.00 0.23s 100.00 Mag.Conc 0167- 31.040- 89.71 0.50NaGN; Barren S0111. and 233. 33 0. 004 3. 17.8 02.0.

Wash. 7 Residue. 100. 00 0.015 I 0.34 12.0 Mag. C.

Example 6" A slimy low grade gold-silver oreirom. Nevada having thetfollowing,composition-z Gold oz. Ali/ton" 01060 Silver oz rig/tone- 2:2

.Zdehour time-of contact for precipitation wasral 76.

lowed. Therpulpr then was treated magnetically and was filtered and washed. The results of the I test were: Mesh Percent Wt.

0.75 Assay, Per Cent Reagent 5.01 Per Oz. per 'Ion Distrib. Consump. 10.61 Product Cent tion. 13. 28 Wt. Lbs/Ton 70. 35

Ag Au Ag Au 131 gem 32315525 5335 232 333 1 080N ON 10 Pul i th' 1: 1 1 ttl a as. one a ps0 isoreare exremeysowse ingan Barren dWash. 281.6

Remit? 166.63 5%; 8.89%? tit? 22.33 333E120. filter wlth h dlfficulty they cannot be handled by ordinary cyanidation on a commercial scale.

Test 2.A comparison test was run similar to Test 1-Comparz'son test.A test charge of the Test 1 except that no precipitant was used and ore was ground with hydrated lime and cyanide after dissolution the pulp was filtered and washed. and then was transferred to an agitating bottle. The residue and the combined filtrate plus wash At pulp density of 34% solids, the pump was water were submitt d for assay, agitated for a 48 hour period after which it was filtered and the residue was washed. The products were submitted for assay. Assay, Oz. Per Cent P d t OPert per Ton Distrib. Reagent Cou- P t R ta 1'0 110 en sump 1011, ercen eagen O11- Wt. Lbs/Ton Product Z W TS 35 Distrib. sumption, Ag Au Ag Au of Au Lbs/Ton Calo.Head 100.00 2.10 0.070 1060010000 Gale. Head 100.00 0.128 100.00 Preg. $0111. and 300.42 0.462 0.0133 66.09 78.57 0.40 NaCN'. Preg. Soln. and 275.97 0.0403 88.24 1.69 NeoN.

Wash. 7.0 02.0. Wash. Residue 100.00 0.71 0.015 33.91 21.43 Residue 100.00 0.015 11.76 20.9 OaO.

Test 2Maynetic charcoal test-The procedure Test 1 w followed of Test 1 was followed exactly except that 8.3 lbs. fixcep that a S mnger cyamde sohftmn and more per ton of dry feed of activated peach pit carbon lme were used" The results Wereprepared as described in Example 1 was added at the start of agitation. After the 48 hours of dissolution and precipitation the carbon was Assay, Oz. Per Cent Per per Ton Distrm Reagentpm separated magnetically and was cleaned once. Product lair g ng g The pulp was then filtered on a laboratory filter Ag Au Ag Au and washed. The results were as follows:

Calc.Head 100.00 2.17 0.060 100. 00 100. 00 v Percent Assay Percent Re s t Mag. Conc 0. 59172.04 7.477 46.76 74.57 0.67 NaON. Pmduct wt, 0 A 1' Dlsmbsumptwn" Bvrrfin and 270.83 0.268 0.0026 33.43 11.90 13.8Ca0. z u/ on OM11 Lbs-W011 Residue 100.00 0.43 0. 00s 19.81 13. 53 9.3 a 0. Cale-Head 100.00 131 100W Mag. Cone 0.46 23.794 83.38 1.66 NaON. Biglrenhsolnaud 296.12 0.0023 5.10 2090210.

Test 4.--Th1s test was similar to Test 3 except Residue 100m 0015 1M3 83 Mai; that the quantity of magnetm carbon was increased to 16.7 lbs. per ton of dry feed- All other Test 3Magnetic charcoal test.The procedure conditions were the same as in Test 3. The results of t 2 was followed t instead of t peach were: pit charcoal, 8 lbs/ton of a magnetic pine wood charcoal was used. ThlS charcoal was prepared A O P C t from 6 parts of 10 +200 mesh pine wood charssay, Z. 81 en Per per Ton Dismh Reagentoom coal, 3 parts of 200 mesh magnetite, 8 parts Product ent Sumption, of Be. sodium silicate and 4 parts of water. LbSJTm After the preparation, drying and activation Ag Au Ag Au i.|

which proceeded as described in the foregoing CaJQHead 100m 2'05 M67 1001010030 examples, the product was calculated to contain g %-33 2 1 15 3. 35 $5.338 6; CNgCN. carbon, 25% magnetite and 25% sodium Wail? an a silicate. Metallurgical results were as follows: Residue 100.00 0.41 0.005 19.97 7.49 16.7 Mag. 0.

Reagent Product Per Cent Assay, gg g g Consump- Wt. Oz. Au/ton f A tion. Example 7 Lbs/Ton A series of tests with various magnetic carbons Cale. Head 100. 00 0.128 100. 00 and a compansontest using the ordinary flotation i Mag Cone 0 41 25 570 81778 L53 NEON method of removmg carbon were carried out on Bavrrenhsoln. and 336.44 0. 0010 2.62 20.0 oeo.

1 low grade Montana gold, ore f m Residue 100. 00 0.020 15.60 8.0 Mag 0.

cipally quartz and pyrite, with various iron oxldes,

micas and clay. The ore assayed as follows: Test 4 C0mpm-ison test, cyanidation,

Gold All/ton" 0130 tatzon and flOtCLiZOTL-ilh a test slmllar to Test 1,

Iron Fe 4 53 a charge of ore was agitated for 48 hours, followa ing .which ordinary activated pine charcoal,

Sulfur S 2.78

Insoluble Insol 35 19 crushed to mesh, was added in the amount of 5.0 lbs. per ton of dry feed. The pulp was The ore ground to the following size: 7 5 agitated for an additional 30 minutes after which silicate and .10 parts of water.

'7 .Reagent 1 Per Cent Product ta? fi gf Distrib. fi

. of Au Lbs/Ton Calc. Head"; 100. 0.127 100.00 2.3 NaCN. Flotfloncen-.. 1.57 6.978 86.33 v 20.0 020. Flat. Tailing 98. 43- 0015 11-69 5.0 Charcoal. Tailing Soln. 433. 33 0. 0006 1. 98 0.54 Fuel-Oil.

0.12 Frothcr.

Example 8 A low-grade clayey ore from Nicaragua was used for a .series of .tests. The ore assayed as "follows:

Gold oz. All/0011-.. 0.12.0 Silver .102. Ag/tom- 0.40

Theore was composed chiefly of quartz, clay and feldspar. Considerable limonite was present and some hematite but the sulfide content was *very low. iPnlps of the ore were extremely viscous and were difficult to settle and filter.

Test '1.--A test .charge cf ore was ground to 4.4% +48 .mesh and 58.2% -200 mesh,and was transferred to wide-mouth agitating :bottle. The pulp was preaerated with lime for .a period :of 2 hours, thencyanide, technical calcium cyanide, and additional lime were added and the pulp was agitated for 24 hoursat 33% solids. After agitation, the pulp was filtered and washed. The filtrate and residue were submitted for assay. The results wereas follows:

' Per cent Reagent Per cent Assay, Oz. Product Distrib. Consumption,

Weight Au/to of Au Lbs/Ton CalmGead 100.-00- 0.125 100.00 Prsz. 50in. and 256.67 0.0448 92.01 OJONaON.

as Residue 100.00, 0.010 f 7.99 20.9030.

Test 2.--'This test was similar :to Test 1 except that after 17 hours of agitation 13.3 lbs/tonof a magnetic charcoal was added, prepared from 25 parts .of -20 +200 mesh peach pit charcoal, mints 200 mesh magnetite, parts of 40 B After drying and activation, as described :in the foregoing examples, the carbon was calculated to have 65% carbon, 26% magnetite and 9% sodium silicate.

12 for'lhours and was then reccvered'magnetlcally. The pulp was filtered on a laboratory filter, washed, and the products assayed. The metallurgical resultsare as follows:

2. on 1 1S 1 Consumption Product LbsJTon is iAu Ag Au Celc.'Hea 1 100.00 0. 408 0. 125 100.00 100.00 Mag. C0110 0.73 46. 21 14. 317 82. 70 83. 75 0.10 NaON. Balg renhsolmand 302. 0.0068 0.0034 5. 04 8. 24 19.2 C30.

as Residue 100. 00 0.050 0. 010' 12. 26 8. 01 13.3 Mag. 0.

Test 3.'Tl'ns testwas a straight froth flotation test. The ore was ground as in'Tests land2 and the pulp "was transferred to a flotation machine and was floated using a higherxanthate, a sodium salt of a dialkyldithiophosphate and a higher alcohol-type frother. Under optimum flotation conditions, only 57.6% of the gold was recovered in a product assaying 1.64.0z. per ton and representing 4.83% of the total Weight of ifeed. It will be noted that the magnetic process recovered about 50% more gold in a much higher grade concentrate.

Example 9 The ore of Example? was used in atest to determine the .effect of a counter-current .flowof magnetic carbon. This was .a locked test.

Three test charges of ore .were treated. The grinding, magnetic separation and thickening were conducted as described in Test .1 of Example 2. 7

The first charge of pulp, at a pulp density of 37% solids, was agitated for 20 hours with'lime, technical calcium cyanide and 9.0 lbs. of the magnetic charcoal used in Test 5 of Example 2. After 20 hours contact the magnetic charcoal was recovered and was submitted for assay. To the ,pulp then were added 9.0 lbsof fresh -magnetic charcoal per ton of feed and an additional 4 hours 'of contact were allowed, after which the magnetic-concentrate was removedand was added to the-second charge of pulp, simulating countercurrent flow of freshcarbon. The second charge of pulp was treated exactly as described above and. the same procedure was followed in treating the third charge. After removal of the second "magnetic concentrate from the third charge, the

concentrate was designated as the circulating carbon and was submitted for assay.

The metallurgical results when compared with those of Example 2 show that a counter-current duced. The low grade of the concentrate from change .No. 1 was due to incomplete removal of naturally-occurring magnetic material before 60 The charcoal remained in contact with the :pulp cyanidation.

. Percent Reagent Charge P PerOent Assay,

- roduct Distnb. Consump- No. Wt. Oz. Ali/ton M Kn fionLbsJTm 1 Cale. Head 1..-... 1100.00 0. 138 100.001

Mag. Conc' 0. 74 7 16.582 88. 61 0.35 NaCN Barren 80111. and Wash: 311. 67 0. 00025 0. 56 9.5 OaO.

Re5idue. M 100.00 0.015 10. 83 9;0,Mag. C 2 100.00 0.145 100. 00

Mag. Comer. .0. 60 20. 822 86. 37 0.30 NaCN- Barren Soln. and WaslL 341. 67 0.0005 1. 18 10.0 020. Residue 100. 00 0.018 12.45 9.0 Mag-C 3...... Oslo. Head 100. 00 0. 141 100. 00

Mag. Cone.--" -0. 22.582 88. 25 0.30NaCN Barren Soln. and ash 360. 00 v 0. 00015 0. 38 10.0020 1 Residue 100.00 0. 016 11.37 9.0 Mag. 0

(Clrcul. Carbon) u, 0. 51 0. 477 (l. 73)

The present invention is of primary importance in the beneficiation of slimy precious metal ores. The process, of course, Works perfectly with ores which are not slimy and which are susceptible to the ordinary oyanidation procedures. In some cases the present invention is of great economic importance even with ores which can be handled technically by other cyanidation procedure. The capital investment of a plant in which the process of the present invention is used is very much smaller than a conventional cyanidation plant with its large filters or decantation tanks. Therefore, when small ore bodies, such as old tailings, piles, and the like are to be treated, the reduced. capital expenditure makes the present process economically feasible where the ordinary cyanidation procedures with their large capital investment could not be used economically. In a broader aspect, therefore, the present invention may be used with ores which can be treated effectively by other cyanidation processes. The preferred modification, however, in which slimy ores are treated constitutes by far the most important practical field of utilization.

We claim:

1. A process of recovering precious metals selected from the group consisting of gold and silver from ores which comprises subjecting the ore to cyanidation, precipitating precious metals by means of activated carbon containing sufficient magnetic material to render the carbon particles magnetic, separating the magnetic precious metal-bearing carbon magnetically.

2. A process of recovering precious metals selected from the group consisting of gold and silver from ores containing the same and associated with magnetic gangue which comprises subjecting the ore to magnetic separation to remove the magnetic gangue therefrom as a magnetic .concentrate, \cyanidin-g the magnetic tailing, precipitating the precious metals therefrom by means of activated carbon containing suflicient magnetic material to render the carbon particles magnetic, separating the magnetic precious metalbearing carbon magnetically.

3. A process of recovering precious metals selected from the group consisting of gold and 14 silver from slimy ores which comprises subjecting the ore to cyanidation, precipitating precious metals by means of activated carbon containing suflicient magnetic material to render the carbon particles magnetic, separating the magnetic precious metal-bearing carbon magnetically.

4. A process of recovering precious metals selected from the group consisting of gold and silver from slimy ores containing the same and associated with magnetic gangue which comprises subjecting the ore to magnetic separation to remove the magnetic gangue therefrom as a magntic concentrate, cyaniding the magnetic tailing, precipitating the precious metals therefrom by means of activated :carbon containing sufiicient magnetic material to render the carbon particles magnetic, separating the magnetic precious metal-bearing carbon magnetically.

5. A process of recovering gold from slimy ores which comprises subjecting the ore to cyanidation, precipitating the gold by means of activated carbon .containing suificient magnetic material to render the carbon particles magnetic, separating the magnetic gold-bearing carbon magnetically.

6. A process of recovering gold from slimy ores containing the same and associated with magnetic gangue which comprises subjecting the ore to magnetic separation to remove the magnetic gangue therefrom as a magnetic concentrate, cyaniding the magnetic tailing, precipitating the gold therefrom by means of activated carbon containing sufiicient magnetic material to render the carbon particles magnetic, separating the magnetic gold-bearing carbon magnetically.

EARL C. HERKENHOFF. NORMAN HEDLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

